1. Field of the Invention
The invention relates to an optical pick-up apparatus for recording data into and reproducing data from an optical disc medium such as a laser disc, a compact disc (CD), and a digital video disc (DVD), and also to a method of carrying out optical pick-up.
2. Description of the Related Art
FIG. 1 illustrates an example of conventional optical pick-up apparatuses to be used for an optical disc apparatus employing an optical disc medium such as CD and DVD. The illustrated optical pick-up apparatus is one suggested in Japanese Unexamined Patent Publication No. 3-214102. In the illustrated optical pick-up apparatus, there are formed two photo-diodes 2A and 2B and an optical integrated circuit (not illustrated) such as an amplification circuit in an optical integrated circuit 1. A prism 3 is mounted on the substrate 1 at a predetermined location above the photo-diodes 2A and 2B. The prism 3 has an inclined side surface 3A. A heat sink 4 having a photo-diode function is also mounted on the substrate 1, and a semiconductor laser 5 is mounted on the heat sink 4.
The semiconductor laser 5 is disposed in facing relation to the inclined surface 3A of the prism 3. An optical system for image-formation including an objective lens 6 is supported by a support device (not illustrated) just above the inclined surface 3A with an optical axis thereof being perpendicularly directed. The objective lens 6 is disposed in facing relation to a recording surface of an optical disc medium 7.
In operation, a laser beam emitted from a front end surface of the semiconductor laser 5 is reflected at the inclined surface 3A of the prism 3, introduced to the optical disc medium 7 through the objective lens 6, and then focused on the recording surface of the optical disc medium 7 to thereby form an image thereon. The laser beam is reflected at the optical disc medium 7, and establishes an optical signal including information stored in the optical disc medium 7. The thus produced optical signal is introduced into the prism 3 through the objective lens 6 and the inclined surface 3A of the prism 3, and received in the photo-diode 2A. Then, the optical signal is reflected at the photo-diode 2A and further reflected at an inner surface of the prism 3, and then received in the photo-diode 2B.
The optical signals detected in the photo-diodes 2A and 2B are coupled to thereby take out information having been stored in the optical disc medium 7 or establish a servo signal for the optical system to control a position of the objective lens 6 for properly introducing the laser beam onto the optical disc medium 7.
The optical output of the semiconductor laser 5 is controlled by detecting a laser beam emitted from a rear end surface of the semiconductor laser 5 with a photo-diode built in the heat sink 4, and carrying out feedback control utilizing the thus detected laser beam.
The optical pick-up apparatus as mentioned above is used, for instance, for CD and CD-ROM. The semiconductor laser 5 employed in the above mentioned optical pick-up apparatus is, for instance, a near infrared semiconductor laser which emits laser beams having a wavelength of 780 nm band, and is used for any optical disc medium. Hence, it would be possible to keep compatibility for any optical disc mediums having different specification without changing basic optical structure of an optical pick-up apparatus.
In response to a demand of increasing a memory capacity, DVD having a memory capacity of a few giga bytes is now being developed. In this DVD, a semiconductor laser which emits laser beams having a shorter wavelength, for instance, a wavelength of 630 nm band, is used as a light source in order to increase a memory capacity.
One of keys for diffusion of the above mentioned DVD is whether it would be possible to keep compatibility with conventional optical disc mediums. It is not possible to process signals in DVD by a semiconductor laser which is used for CD and emits laser beams having a wavelength of 780 nm band. In contrast, it is not possible to process signals in CD by a semiconductor laser which is used for DVD and emits laser beams having a wavelength of 630 nm. Accordingly, in order to keep compatibility between DVD and conventional mediums such as CD and CD-ROM, it would be necessary to additionally employ a semiconductor laser which emits laser beams having different wavelengths.
However, a wavelength to be used is determined by one semiconductor laser in the above mentioned optical pick-up apparatus, and hence it would be necessary to additionally construct an optical pick-up apparatus including other semiconductor laser or semiconductor lasers, in order to have an optical pick-up apparatus which emits laser beams having different wavelengths. For instance, it would be necessary to mount two different optical pick-up apparatuses on an optical disc apparatus. Accordingly, two sets of elements such as an objective lens and a photo-diode for constituting an optical disc apparatus would be necessary to be prepared, which is accompanied with problems of increased fabrication costs because of an increase in the number of elements, and an increase in size of an optical head.
Japanese Unexamined Patent Publication No. 62-196880 has suggested a semiconductor laser apparatus comprising a photo-detector formed on a semiconductor substrate, a prism fixed on the photo-detector and having a half-transmissive reflective surface and at least one reflective surface, and a semiconductor laser fixed on the semiconductor substrate. A laser beam emitted from the semiconductor laser and reflected at the half-transmissive reflective surface is used as an irradiation beam. A laser beam entering and transmitting through the half-transmissive reflective surface and reflected at the reflective surface is detected by the photo-detector.
Japanese Unexamined Patent Publication No. 62-197931 has suggested a focus detector comprising a semiconductor laser fixed on a semiconductor substrate, a prism fixed on the semiconductor substrate, a first half-transmissive reflective surface formed in a surface of the prism which faces the semiconductor laser, a second half-transmissive reflective surface of the prism which makes abutment with the semiconductor substrate and to which a laser beam having been transmitted through the first half-transmissive reflective surface is introduced, a first photo-detector including three photo-detecting sections arranged in a certain direction, the first photo-detector making facing relation with the second half-transmissive reflective surface, and a second photo-detector including three photo-detecting sections arranged in a certain direction, the laser beam reflected from the second half-transmissive reflective surface entering the second photo-detector. An optically recording medium is irradiated by the laser beam emitted from the semiconductor laser and reflected from the first half-transmissive reflective surface. The laser beam reflected from the optically recording medium and transmitting through the first half-transmissive reflective surface is focused after having been reflected at the second half-transmissive reflective surface but before entering the second photo-detector. A focus error signal of the optically recording medium can be obtained by comparing a first sum to a second sum wherein the first sum is a sum of detecting signals emitted from the photo-detecting sections disposed at opposite ends in the first photo-detector and a detecting signal emitted from the photo-detecting section disposed at the center in the second photo-detector, and wherein the second sum is a sum of a detecting signal emitted from the photo-detecting section disposed at the center in the first photo-detector and detecting signals emitted from the photo-detecting sections disposed at opposite ends in the second photo-detector.
Those apparatuses suggested in the above mentioned Japanese Unexamined Patent Publications have the same problems as those of the optical pick-up apparatus having been explained with reference to FIG. 1.